linux-kernel - Re: question about save_xstate_sig()

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Date:	Tue, 27 Jan 2015 15:27:39 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: question about save_xstate_sig() - WHY DOES THIS WORK?

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On 01/27/2015 02:40 PM, Oleg Nesterov wrote:
> On 01/26, Rik van Riel wrote:
>> 
>> On 01/24/2015 03:20 PM, Oleg Nesterov wrote:
>> 
>>> Now the questions:
>>> 
>>> - This doesn't hurt, but does it really need __thread_fpu_end?
>>> 
>>> Perhaps this is because we do not check the error code returned
>>> by __save_init_fpu? although I am not sure I understand the
>>> comment above fpu_save_init correctly...
>> 
>> Looking at the code some more, I do not see any call site of 
>> save_init_fpu() that actually needs or wants __thread_fpu_end(), 
>> with or without eager fpu mode.
> 
> Yes. But probably it is needed if __save_init_fpu() returns 0. But
> this is minor, __thread_fpu_end() doesn't hurt correctness-wise if
> !eager.
> 
>> It looks like we can get rid of that.
> 
> Agreed, but probably this needs a separate change.
> 
>>> - What about do_bounds() ? Should not it use save_init_fpu() 
>>> rather than fpu_save_init() ?
>> 
>> I suppose do_bounds() probably should save the fpu context while 
>> not preemptible,
> 
> plus it also needs the __thread_has_fpu() check. Otherwise
> fpu_save_init() can save the wrong FPU state afaics.
> 
>> but that may also mean moving conditional_sti() until after
>> save_init_fpu() or __save_init_fpu() has been called.
> 
> Agreed, this can work too.
> 
>>> - Why unlazy_fpu() always does __save_init_fpu() even if 
>>> use_eager_fpu?
>>> 
>>> and note that in this case __thread_fpu_end() is wrong if 
>>> use_eager_fpu, but fortunately the only possible caller of 
>>> unlazy_fpu() is coredump. fpu_copy() checks use_eager_fpu().
>>> 
>>> - Is unlazy_fpu()->__save_init_fpu() safe wrt
>>> __kernel_fpu_begin() from irq?

It looks like it should be safe, as long as __save_init_fpu()
knows that the task no longer has the FPU after __kernel_fpu_end(),
so it does not try to save the kernel FPU state to the user's
task->thread.fpu.state->xstate

The caveat here is that __kernel_fpu_begin()/__kernel_fpu_end()
needs to be kept from running during unlazy_fpu().

This means interrupted_kernel_fpu_idle and/or irq_fpu_usable
need to check whether preemption is disabled, and lock out
__kernel_fpu_begin() when preemption is disabled.

It does not look like it currently does that...

>>> I mean, is it safe if __save_init_fpu() path is interrupted by 
>>> another __save_init_fpu() + restore_fpu_checking() from 
>>> __kernel_fpu_begin/end?
>> 
>> I got lost in the core dump code trying to figure out whether
>> this is safe or broken. I'll need some more time to look through
>> that code...
> 
> It is called indirectly by regset code, see
> xstateregs_get()->init_fpu().
> 
> The coredumping task can't return to user-mode and use FPU in this
> case, so this is not that bad. Still
> unlazy_fpu()->__thread_fpu_end() is wrong if eager.
> 
> And I forgot to mention, the "else" branch in unlazy_fpu() makes no
> sense. And note that save_init_fpu() and unlazy_fpu() is the same
> thing (if we fix/cleanup them).

I was wondering why there were several functions doing essentially
the same thing...

> Oh. I'll try to finish my cleanups and send them tomorrow. Unless
> you do this ;)

If you tell me what you would like to see done, I'd be more than
happy to do it :)

I can certainly merge unlazy_fpu() and save_init_fpu() into the
same function, but I am not sure whether or not it should call
__thread_fpu_end() - it looks like that would be desirable in some
cases, but not in others...

- -- 
All rights reversed
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